1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) transmission method of downlink pilot channel in a CDMA mobile communication system, a CDMA mobile communication system, a base station and a mobile station.
2. Description of Related Art
FIG. 1 is a schematic diagram illustrating a spreading code arrangement in a CDMA mobile communication system.
As illustrated in FIG. 1, cell division is usually implemented by spreading codes with a long period (called “scrambling codes” from now on) that are uniquely assigned to cells each, and channel division is implemented by spreading codes with a short period (called “channelization codes” from now on) that are uniquely assigned to channels in the cells each.
Although the channelization codes usually consist of codes orthogonal to each other, the number of such codes is limited, and hence, a shortage of the channelization codes can take place. In such a case, that is, when a set of orthogonal codes is exhausted, a new channelization code set is generated by assigning a plurality of scrambling codes to each cell. This will be described with reference to FIG. 1. In cell #1, although the channelization code set #1 corresponding to channels CH#1+#N employs a scrambling code SCCODE #1, the channels CH#N+1 and onward use a scrambling code SCCODE #2 to create a new channelization code set #2.
Next, referring to FIGS. 2 and 3, an arrangement of downlink pilot channels and an example of transmission power of each channel in a conventional technique will be outlined.
FIG. 2 is a diagram illustrating an arrangement of the downlink pilot channels on the conventional technique. In FIG. 2, the height of each channel schematically represents the symbol rate of the channel, which means that a thicker channel has a higher symbol rate. The CDMA mobile communication system carries out orthogonalization by spreading data symbols of various channels using the channelization codes to transmit these channels at the same transmission rate. Here, the same transmission rate after the spreading is referred to as “chip rate”, and the transmission rate of the data symbols before the spreading is referred to as “symbol rate”. Generally, a system with a chip rate of N chips/second can contain the orthogonal code set of the cells in a single sequence if the total symbol rate of the channels is equal to or less than N.
The downlink pilot channel CH#1 is provided for the purpose of obtaining phase information used for demodulating the downlink traffic channels, detecting a received path, and obtaining interference power for traffic channels to carry out the transmission power control. The pilot channel has the minimum symbol rate provided by the system (the CH 1 is denoted as a thinnest channel in FIG. 2) because it either undergoes data modulation by a known pattern or does not undergo any data modulation at all.
FIG. 3 is a diagram illustrating transmission powers of downlink channels in the conventional technique. In FIG. 3, the height of each channel schematically denotes the transmission power of the channel. Thus, a higher channel has higher transmission power. Since the pilot channel CH#1 is utilized for the purposes mentioned above, it is usually transmitted at the transmission power higher than the transmission power of the other traffic channels as illustrated in FIG. 3.
To achieve interference power measurement for the transmission power control at high accuracy, it is necessary to generate many samples of a signal used for the measurement. However, since the conventional pilot channel is provided with the minimum symbol rate of the system, it is impossible to obtain sufficient samples, presenting a problem of hindering high measuring accuracy.
In addition, when using a plurality of orthogonal code sets in the downlink, although the channels belonging to the same orthogonal code set do not interfere with one another on the same received paths, they can interfere on the other paths. On the other hand, channels belonging to different orthogonal code sets can interfere to each other regardless of whether on the same paths or the other paths. Accordingly, the interference power received by the traffic channels differ depending on the orthogonal code sets to which the traffic channels belong. However, since the pilot channel is provided for only one orthogonal code set as illustrated in FIG. 2, there arises a problem in that the channels in the other orthogonal code sets prevent accurate interference power measurement for the transmission power control.
Moreover, since the pilot channel is an overhead channel without transmitting any information, transmitting it beyond necessary transmission power will bear upon the other channels, presenting a problem of reducing channel capacity. On the other hand, the interference power measurement can be achieved even if the transmission power is zero by measuring the variance of the channel, and when using the pilot channel for the purposes other than the interference power measurement, it usually does not matter whether it belongs to the same orthogonal code set or not.